1. Field of the Invention
The present invention relates to a semiconductor laser device used as a pumping light source of a solid state laser such as an Nd doped YAG laser and a Yb doped YAG laser, a Yb doped fiber laser, an Er doped fiber amplifier, or the like.
2. Description of the Related Art
In order to provide a semiconductor laser device which is highly reliable when operated at high power, InGaAsP which has a high optical damage threshold and which can decrease the electrical resistance is adopted as an active layer. The InGaAsP active layer is sandwiched between an InGaP guide layer and an allegiant clad layer (see, for example, Hirofumi Kan and three others, “High Power Laser and Applications for Energy Source”, Trans. IEE Japan, Vol. 125-C, No. 2, 2005, pp. 187-194).
However, in recent years, in order to decrease the power consumption, the necessity of a semiconductor laser having high electrical transduction efficiency is growing. Improvement in internal quantum efficiency and improvement in slope efficiency by decreasing losses have almost reached a limit. There is still a problem of lowering the operating voltage. However, if, like in a conventional semiconductor laser having an oscillation wavelength of 808 nm, an InGaP layer which is lattice-matched to GaAs at a thickness of 120 nm is included between an active layer and a clad layer, there is a problem in that the operating voltage is raised.